A simple spherical ball and socket connection provides a coupling that is rotationally compliant and axially stiff . A coupling of this type is used as an end fitting for linkages or rods or struts where axial forces must be rigidly transferred, and rotations are relatively unconstrained. The ball and socket end connection is however characterized by large friction losses, making it undesirable for low power, precision coupling and in particular closed loop feedback controls, which can become unstable when too much stick-slip motion is present. Pivot flexures, like the coupling 4 shown in FIG. 4, are generally more useful in those applications. This device operates in a completely linear fashion by means of elastically deflecting flexure blades 4a that produce the desired linkage 4c end rotation with a fixed end or other linkage 4d. Rotational compliance about the two axes orthogonal to the link axis (direction 4b in FIG. 4) is desired thus the term biaxial is often used to describe these flexures. Some rotational compliance about the link axis is also achieved with this type of flexure although it is not critical in most applications
Even though blade type pivot flexures have been used successfully, they possess certain performance limitations, in particular weak axial load carrying capacities. To achieve low rotational stiffness, the interleaved blades 4a must be slender which makes them susceptible to buckling under compressive or axial loads, in the direction 4b. To increase their load bearing capacity, the blades 4a conceivably can be made thicker, which makes them less flexible and increases rotational stiffness reducing the coupling's effectiveness. The goal is minimal rotational stiffness, which requires the thinnest possible blades but maximum axial strength.